This invention relates generally to systems for packet switched communications networks, and, more particularly, relates to apparatus and methods utilizing a high-speed, multiprocessor packet switching configuration.
A wide range of telecommunications network configurations have been proposed or implemented in recent years for providing communication between data handling or data communications devices. In particular, packet switching systems were developed to fulfill a demand for low cost data communications in networks that provide access to host computers.
In conventional packet switched systems, digital data cells or packets are transmitted to a selected destination by a terminal, computer, applications program or other data handling device. The destination can be another data handling or data communication apparatus or system. In many packet switched systems, special-purpose computers are employed as packet switching processors--i.e., communication processors adapted to direct packets along the network.
One class of packet switching systems utilizes predetermined paths through the network, in which packets generated by a plurality of users share link and switch facilities as the packets travel over the network. In these systems, the packets must be stored at nodes between transmission links until the packets can be forwarded along the appropriate destination link. This class of data transmission system is referred to as virtual circuit or connection-oriented transmission.
Another class of packet switching systems utilizes connectionless transmission, which requires no initial connection for a data path through the network. In these systems, individual data cells or packets, including a destination address, are routed through the network from source to destination via intermediate nodes.
The virtual circuit system is utilized in a public network established by Telenet Communications Corporation. This system employs a two-level hierarchy to route data packets. One level of the hierarchy is a network having a plurality of hubs and nodes, each of which utilizes a cluster of switches. The second level includes smaller area networks having trunks, access lines and clustered lower level switches connected to each hub. The Telenet system utilizes the X.75 protocol promulgated by the International Telegraph and Telephone Consultative Committee of the International Telecommunications Union (CCITT) as an interface for connecting computers to a packet-switched network. The protocol is structured in a three-layered configuration, and the layers are referred to as the physical level, the frame level, and the packet level. Routing paths in the Telenet system are determined by a packet switching processor, which utilizes routing tables indicating available links from each hub. The conventional packet switching processor used in the Telenet system includes a main memory unit, line processors that control access to user lines, and a central processing unit (CPU) that controls routing at the packet level. The CPU employs a table of trunk-to-trunk active virtual circuits to identify appropriate hubs and virtual circuits for connecting access-requesting users. In the Telenet system, each user transmitting data across the network must first write its data packets into main memory, via a bus. The line processors associated with each user compete for access to the bus, in accordance with a conventional token-passing configuration under the control of an arbitration unit.
Another form of conventional virtual circuit packet switching system utilizes multiple bus request chains having different predetermined priorities. Each chain in the system employs an arbitration scheme similar to that described above.
In yet another example of a conventional packet switching network, a user seeking access to the bus must first transmit a selected signal pattern onto the arbitration bus. Several line units can drive the bus simultaneously in this configuration. The driving line units periodically read these signal patterns and, based on the priorities of other requesters, determine whether to maintain or abandon their respective requests. This process continues unit an arbitration unit declares a winner.
Conventional packet switching systems, however, suffer significant limitations in communications bandwidth and speed, resulting from their bus arbitration schemes and the requirement of writing packets into main memory across a bandwidth-limited bus.
For example, in systems utilizing token-passing arbitration, a requesting unit must maintain its request and remain idle until the token is passed along the chain to that unit, even if that unit is the only processor requesting access to the bus. Time and communications channel capacity are squandered while the token is passed from user to user. As a result, certain conventional systems of this type are limited to a data transmission rate no greater than approximately 164 megabits/second.
One object of the invention, therefore, is to provide improved packet switching methods and apparatus enabling enhanced packet transmission rates.
Another object of the invention is to provide packet switching methods and apparatus that afford high bandwidth packet transfer.
A further object of the invention is to provide packet switching methods and apparatus enabling multiple requesting processors to receive substantially instantaneous access to the packet switched network.
Another object of the invention is to provide such methods and apparatus capable of handling a wide range of existing and proposed packet protocols.
Still another object of the invention is to provide packet switching methods and apparatus that enable substantially simultaneous handling of packet switching operations and applications program processes.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.